[Python-Dev] deja-vu .. python locking

"Martin v. Löwis" martin at v.loewis.de
Tue Sep 19 20:13:29 CEST 2006

Martin Devera schrieb:
> I measured it. Lock op in futex based linux locking is of the same
> speed as windows critical section and it is about 30 cycles on my
> P4 1.8GHz in uncontented case.
> As explained in already mentioned http://www.linuxjournal.com/article/6993
> it seems due to pipeline flush during cmpxchg insn.
> And there will be cacheline transfer penalty which is much larger. So
> that mutex locking will take time comparable with protected code itself
> (assuming fast code like dict/list read).
> Single compare will take ten times less.
> Am I missing something ?

I'll have to wait for your revised algorithm, but likely, you will need
some kind of memory barrier also, or else it can't work in the
multi-processor case.

In any case, if to judge whether 30 cycles is few or little,
measurements of the alternative approach are necessary.


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